In this proposal, we have designed biochemical agents to inhibit transferrin-mediated uptake of radioisotopes. Our objective is to deliver these agents specifically to normal hepatocytes in order to reduce background radiation for contrast enhancement of positive images in the evaluation of hepatic tumors. To achieve this objective, 111Indium will be loaded onto transferrin to deliver 111Indium to both normal and malignant liver cells via internalization by receptor-mediated endocytosis. Release of isotopes from transferrin requires acidification within intracellular (endosomal) compartments. Because certain amines, e.g. primaguine, are capable of inhibiting endosomal acidification, these agents can be used to prevent transferrin mediated-radionuclide uptake. Our strategy for enhancement of contrast by selective suppression of radiolabel uptake in normal hepatocytes is based on a functional difference between normal and malignant hepatocytes: the ability of normal cells and inability of the majority of malignant hepatocytes to bind and internalize galactose-terminal (asialo)-glycoproteins. Agents coupled to such proteins can be specifically targeted to only normal hepatocytes with subsequent release of the active agent from its protein carrier. We hypothesize that if primaguine were chemically coupled to an asialoglycoprotein carrier, in the presence of 111Indium-transferrin, suppression of 111 Indium uptake by the primaquine conjugate would be limited to only normal (asialoglyco-protein receptor (+)) hepatocytes resulting in enhanced contrast of tumor uptake of radionuclide. We propose to study this approach using two model systems: (1) in vitro cultures of normal and malignant hepatocytes (2) rats bearing implanted hepatomas.